Just leave it for a day... Was expeeting wind shift from south but not that strong as it turned straight to the bow which was held by only one mooring line. And then the transom had a long french kiss with pontoon. Luckily neighbours noticed "jumped out" fender and did adjust the situation but the damage was already done.

Sanding and sanding after polyester filler with fiber glass.

Sanding
Sanding after polyester filler with fiber glass

Adding polyester putty (could have gone without it but I felt I need to experiment) and sanding again.

Adding polyester putty
Sanding

Applying gelcoat and oversanding.

Adding polyester putty
Sanding

This was my first serious try and I will admit it was not perfect. And I had to sand a bit more and to apply more coats of gelcoat. Main thing is to keep it waterproof :)